Cellular communication systems supporting mobile communications have become ubiquitous and in particular second generation cellular communication systems such as the Global System for Mobile Communication and third Generation cellular communication systems such as the Universal Mobile Telecommunication System (UMTS) have become widespread.
In order to provide improved communication services and increased efficiency, cellular communication systems are continuously developed and enhanced. For example, currently, the 3rd Generation Partnership Project (3GPP) standards body is in the process of standardising improvements to GSM and UMTS known as Long Term Evolution (LTE)/System Architecture Evolution (SAE).
Furthermore, there is a strong desire for new communication standards and enhancements to be implemented while allowing existing functionality to still be used. Accordingly, 3GPP has standardised a network architecture wherein different radio access networks are coupled to a common core network. For example, a 3GPP communication system may comprise both a UMTS Terrestrial radio access network (UTRAN) and a GSM EDGE (Enhanced Data rates for GSM Evolution) radio access network coupled to a common core network. In addition, 3GPP is standardising LTE/SAE to allow an LTE based radio access network known as Evolved-UTRAN (E-UTRAN) to be coupled to the 3GPP core network.
However, a challenge for such an approach is that of how provide efficient and practical interworking of the different radio access networks in order to allow communication services to be effectively provided to user equipments. Furthermore, this problem is complicated by the communication system needing to support both circuit switched services, such as for conventional (GSM) voice calls, as well as packet switched services, such as e.g. data services for Internet access.
In particular, the core network comprises circuit switched network elements, such as 3GPP Mobile Switching Centres (MSCs), for switching circuit switched services as well as packet switched network elements, such as Serving GPRS Support Nodes (SGSNs), for routing packet switched services. Indeed, most deployed 3GPP core networks comprise the required functionality for supporting both packet and circuit switched services of UTRANs and GERANs. Furthermore, this functionality is also suitable for supporting packet services of E-UTRANs. However, it has been the intention that E-UTRAN circuit switched like services will exclusively be supported by new core network functionality known as the IP Multimedia Subsystem (IMS). IMS is an architectural framework for delivering Internet Protocol (IP) multimedia to mobile users. IMS is being standardized by 3GPP and is part of the vision for evolving cellular networks towards an IP based system.
However, as IMS requires introduction of new functionality with an associated cost impact, there has been a desire to allow circuit switched services to be supported without the requirement of IMS functionality.
Specifically, for a system comprising both an E-UTRAN and a UTRAN and/or GERAN it has accordingly been proposed that a user equipment camped on the E-UTRAN may switch to the UTRAN/GERAN in order to set up a circuit switched call. Specifically, 3GPP has initiated a new work item called Circuit Switch Fallback with the objective of specifying the architecture enhancements and functionality required to enable fallback from E-UTRAN access to UTRAN/GERAN circuit switched domain access. Using this functionality, voice and other circuit switched services are realized by reuse of existing circuit switched infrastructure. The architecture for this feature is being defined in 3GPP technical specification TS 23.272, “Circuit switched Fallback in Evolved Packet System”.
A specific problem of this approach is how to ensure a reliable and efficient switching of the destinations for incoming paging and packet session setup requests for the user equipments. Specifically, although a user equipment may be simultaneously registered with an E-UTRAN and a UTRAN/GERAN in the sense that the radio access networks may have information of the user equipment being contactable in the system, it can only be reached through one radio access network at any given time. Thus, whereas the user equipments may be registered with more than one radio access network, a user equipment will typically only be able to camp on one radio access network at a time. In particular, a single radio user equipment in idle mode will only be able to monitor for paging and control messages in a single radio access network and accordingly it is important that the system keeps track of which radio access network the user equipment is currently camped on.
Accordingly, if a user equipment is camped on the E-UTRAN and seeks to initiate a circuit switched call, the circuit switched fallback feature allows this circuit switched call to be set up via the UTRAN/GERAN. However, as the user equipment switches to the UTRAN/GERAN, it is required that the routing/paging information in the system is switched from pointing to the E-UTRAN to pointing to the UTRAN/GERAN.
It has been suggested that this can be achieved by preserving packet switched bearers before setting up any circuit switched domain calls or services. Specifically a standards contribution has proposed an approach for call establishment that requires the user equipment to first establish a packet data session via the E-UTRAN, then handing this packet session over to the packet switched domain of the GERAN/UTRAN before finally setting up a circuit switched call in the UTRAN/GERAN.
However, although this approach may reduce the impact on a legacy MSC it also has a number of disadvantages. For example, it is a relatively complex process requiring additional signalling and operations to be performed by the user equipment. Furthermore, it substantially increases the time it takes to set up a circuit switched call. Indeed, a substantial delay is incurred as a dummy packet session must be established and handed over before the set up of the circuit switched call can be initiated.
Hence, an improved communication system would be advantageous and in particular a system allowing increased flexibility, reduced call setup times, increased backwards compatibility, improved support for circuit switched services, improved support for different radio access networks, facilitated operation and/or improved performance would be advantageous.